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Apple PC Compatibility Card 12" 100MHz Overclock

Big Ben

Well-known member
Hello,

This is a work in progress thing, since it’s not stable.

The Pentium on the compatibility pc card is a P54C socket 5 processor.
It runs on a 66MHz bus.

The Pentium has two inputs pins allowing the internal clock multiplier to be configured. Those pin are BF0 and BF1.

Both are pulled up to the 3.3V rail. This configuration is tue default 2/3 ratio, meaning the 66MHz system bus is multiplied by 3/2 or 1.5, giving us an almost 100MHz core clock.

On the PC Compatibility Card there is a marking for a R55 on the backside, just around CPU pin.

By soldering a small resistance (330 ohms max, as recommended by intel datasheet), you will obtain a x2 multiplier giving you a 133MHz clocked CPU.



Why you shouldn’t do this:
- PCI PC Compatibility are rare you don’t want to ruin them
- It’s designed for a 100MHz, the CPU is not meant to be overclocked not the power stage is designed for a 133MHz CPU
- It needs cooling as the CPU will get hotter
- It’s not tested enough
- It’s not stable enough

Unfortunately, I haven’t identified the resistor emplacement for the BF1 pin, pulling down BF1 will allow a x3 multiplier, the maximum, for a 200MHz CPU, the maximum without going with a P54C overdrive.

More to come…
 

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alectrona2988

Well-known member
I'm not too familiar with these cards, could you swap the CPU out for a higher-rated chip and mess with the resistors and such from there?
 

Byrd

Well-known member
Thanks Big Ben for looking into this! 133mhz is good enough for me.

@k24a1 a soldering master could remove the socket, but that’s hundreds of pins on a very packed PCB - good chance of failure. There may also be firmware limitations on CPU support.
 

Big Ben

Well-known member
There may also be firmware limitations on CPU support.

None I’m aware of. The card should support any P54C socket 5 CPU.


I'm not too familiar with these cards, could you swap the CPU out for a higher-rated chip

Yes but as mentioned by Byrd, it requires the original CPU to be removed, that’s a 298 through hole pins monster to get out of the card with damages.

I’m tempted to do so, but I don’t want to ruin the card and I need did something like this. And I got no similar stuff to train on…

So for now the goal is to make is stable at 133MHz then we will see.
 

Big Ben

Well-known member
Replacing the original thermal pad, with a new thick one (because it’s the only thing I had) I made it stable with a fan blowing on the heatsink.

The ceramic package is our enemy here, but it seems 166MHz also existed with this packaging.
So I’m tempted to try the 2.5 multiplier. Could I push my luck up to 200MHz? We will see! 😁

I also ordered a socket 5 to be able to swap the CPU with a better one.
 
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